Physics 202 MWF 12:40 Spring 1993 (Ford)
Exam II Chapts. 28-31 in SZY College Physics 7e


Show all your work. Partial credit will be given if earned. Write your answers in the blanks provided.


(18 pts) 1. The circuit shown in the sketch has an emf , two resistors R₁ = 3 and R₂ = 6 , a capacitor with capacitance C = 5.0 × 10^-4 F, and a switch. The capacitor is initially uncharged. At time t = 0 the switch is closed.

a) Just after the switch is closed, what are the currents i₁ through R₁ and i₂ through R₂?

Ans. i₁ = __6 A__

i₂ = __3 A__

b) When the charge on the capacitor is q = 6.0 × 10^-3 C, what are the currents i₁ through R₁ and i₂ through R₂? (Recall that C = Q/V.)

Ans. i₁ = __6 A__

i₂ = __1 A__


(20 pts) 2. In the circuit shown in the sketch the voltage across the resistor R₂ is 12 V.

a) What is the emf of the battery?

Ans. __18 V__

b) What is the current through the resistor R₃?

Ans. __3 A__


(20 pts) 3. Two long, straight, parallel wires separated by 0.20 m carry currents in opposite directions as shown in the sketch. A small particle with charge q = +5.0 × 10^-4 C is moving between the wires. When the particle is midway between the two wires, the force F that the magnetic field of the wires exerts on it has magnitude
3.0 × 10^-5 N and is directed to the left. At this instant, what are the magnitude and direction of the velocity of the particle? (Show the direction of the velocity in the sketch below. )

Ans. __6 × 10³ m/s__


(21 pts) 4. A long coaxial cable consists of a solid conductor of radius a, supported by insulating disks on the axis of a tube with inner radius b and outer radius c. If the central conductor and tube carry equal currents I in opposite directions, derive an expression for the magnetic field in the following regions. (To get credit for each part of the problem you MUST show clearly how you use Ampere's law to arrive at your answer.)

a) r < a (within the inner conductor)

Ans. __B = µ₀Ir/2a²__

b) a < r < b (in the air gap between the conductors)

Ans. __B = µ₀I/2r__

c) r > c (outside both conductors)

Ans. __0__


(21 pts) 5. Consider the circuit shown in the sketch. Note that two currents are given.

a) What is the resistance R?

Ans. __3 __

b) What is the emf ?

Ans. __12 V__

c) What is the potential difference V_ab, and which point is at higher potential?

Ans. V_ab= __+24 V__

which point is at higher potential (a or b?) __a__